The bearing of a wind turbine has a clearance of approximately −0.1 to 0.2 mm In order to control the lifetime and air-gap of the generator it is advantageous to control the clearance. The problem is that the inner ring of the bearing heats up and expands which leads to reduced lifetime of the bearing. The reason for the heat up is that the inner ring of the bearing together with the shaft structure has a small mass/stiffness compared to the mass/structure of the outer ring. This means that the average temperature of the inner ring is higher than the average temperature of the outer ring and structure around the outer ring.
The bearing of a wind turbine is normally not cooled. However, it is generally known to cool a bearing by an integrated oil lubrication system where the oil is cooled and pumped around in the bearing assembly. The exchange of oil is limited and not sufficient in order to cool down a large main bearing for a direct drive wind turbine. Furthermore, oil lubrication systems for integrated cooling are complex and there is always the risk of oil leakage in the wind turbine which should be avoided.
For grease lubricated bearings no cooling systems are known.